A significant part of the cost of a crystalline silicon photovoltaic module may be attributed to the cost of the silicon substrates used. Therefore, working with thinner substrates may lead to a cost reduction of the module. For thinner substrates (e.g. for substrates with a thickness of less than 180 micrometer, e.g. down to 5 micrometer) there is a need for suitable fabrication processes, because thin substrates are more susceptible to breakage and warpage as compared to thicker cells. Both breakage and warpage can substantially impact the yield of a production process and therefore the cost of photovoltaic cells and modules.
Methods have been proposed for fabricating photovoltaic modules comprising thin back-contact photovoltaic cells, wherein after front side processing the partially processed substrates are bonded to an optically transparent carrier (superstrate) by means of an adhesive, and the rear side processing is done afterwards (module-level processing). In such methods the rear side cell processing is done while the substrates are bonded on a supporting structure, thus reducing the risk of breakage and warpage.
After bonding of the substrates to the carrier, rear side processing of the substrates is done. Because of the presence of a carrier (superstrate, e.g. glass) and an adhesive, there are a number of restrictions for the rear side process steps, for example related to the process temperature and the use of certain chemicals.
Process temperatures for the module-level rear side processing are typically limited to 200° C. or less. This means that widely adopted schemes for surface passivation, emitter formation and/or Back Surface Field (BSF) formation for high-efficiency cells cannot be used. A low-temperature alternative is the a-Si:H/c-Si heterojunction cell, wherein the emitter and a BSF may be formed by PECVD deposition of a doped amorphous silicon layer. A thin high-quality intrinsic a-Si:H layer (passivation layer) may be provided in between the doped a-Si:H layer and the crystalline silicon substrate, for improved passivation of interface defects.
Silicone-based adhesives are typically used to bond the substrates to the carrier. The adhesive is preferably applied on the carrier over an area larger than the area of silicon substrates, i.e. extending outside the edges of the silicon substrates, to enable homogenous and stable bonding.
In the areas not covered by the substrates the adhesive is directly exposed to the plasma during the intrinsic a-Si:H passivation layer PECVD deposition. This results in a degradation of the passivation quality. This degradation may be caused by outgassing of silicone molecules of low molecular weight and/or etching of silicone molecules during amorphous silicon deposition, and/or it may result from an interaction between volatile compounds escaping from the adhesive with precursors used in the amorphous silicon deposition process.
As reported by V. Steckenreiter et al. in “Qualification of encapsulation materials for module-level-processing”, Solar Energy Materials and Solar Cells 120 (2014), 396-401, the quality of the amorphous silicon surface passivation may be improved by outgassing the silicone adhesive in the absence of a plasma immediately prior to the amorphous silicon deposition. The effect of such pre-outgassing treatment depends on the type of adhesive used. It is a disadvantage of this approach that it only works in certain cases depending on the silicone composition. The pre-outgassing treatment is done in the reactor used for a-Si:H deposition, just before a-Si:H deposition. Performing the pre-outgassing in the same reactor as the a-Si deposition may lead to contamination of the chamber.